Chemistry Reference
In-Depth Information
3000
2500
1000
500
2000
0
4
6
8
10
pH
1500
1000
500
0
300
400
500
600
700
800
Wavelength (nm)
Figure 6.18.
Spectrum of O
3
Mn(OH)
−
(
) in argon-purged 0.025 M phosphate, 0.1 M
ethanol, pH 4, spectrum of
MnO
2−
(solid curve) in 0.1 M NaOH, and Mn(VI) adduct
spectra of the
• −
CO
2
radical (
) in N
2
O-saturated 2 mM formate and of the
tert
-butyl
alcohol radical (
) in N
2
O-saturated 0.1 M
tert
-butyl alcohol solution at pH 9.4 (≈1 mM
borate buffer). These spectra recorded 2-10 µs after the pulse. The inset shows the pH
dependence of the Mn(VI) extinction coefficient at 610 nm in 0.025 M phosphate buffer,
0.1 M ethanol (adapted from Rush and Bielski [183] with the permission of the Ameri-
can Chemical Society).
O Mn OH
(
)
−
H MnO
+
+
2
−
.
(6.62)
3
4
The solid line in the inset of Figure 6.18 was calculated using the p
K
62
value
of 7.4 [183]. The kinetics of the disproportionation of the
MnO
2−
ion in acidic
solution has been performed [193]. The decay of the
MnO
2−
ion followed a
pseudo-first-order rate law, which resulted in the formation of
MnO
−
ion. The
formation of the permanganate ion obeyed second-order kinetics.
The spectra of Mn(V) under different conditions are presented in Figure
6.19 [183]. It is clear from Figure 6.19a that the spectra were sensitive to pH
in the range of 0.01-10.0 M NaOH. This sensitivity was utilized to estimate the
acid dissociation constant of Mn(V):
O Mn OH
(
)
2
−
H MnO
+
+
3
−
p
K
≈
13 7
.
.
(6.63)
3
4
63
The spectrum of the Mn(V) ester was also obtained by reducing Mn(VI)
by
tert
-butyl radicals (Fig. 6.19b) [183]. In the reduction process, an intermedi-
ate (
A
) was initially formed, which subsequently decayed by first order to the
final product of Mn(V) (
B
):
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